This section provides background information related to the present disclosure and is not necessarily prior art.
In an attempt to reduce the quantity of NOx and particulate matter emitted to the atmosphere during internal combustion engine operation, a number of exhaust aftertreatment devices have been developed. A need for exhaust aftertreatment systems particularly arises when diesel combustion processes are implemented. Typical aftertreatment systems for diesel engine exhaust may include one or more of a diesel particulate filter (DPF), a selective catalytic reduction (SCR) system, a hydrocarbon (HC) injector, and a diesel oxidation catalyst (DOC). Typical SCR systems include a reductant delivery system for injecting a reductant (e.g., urea) upstream of an SCR catalyst.
More recently, reactors have been provided to generate ammonia gas from liquid reductant. Compared to liquid urea, gaseous ammonia is more reactive, more easily dispersed uniformly in the exhaust stream, and is more active over a wider temperature range. Therefore, the use of ammonia gas in an SCR system can improve the efficiency and effectiveness of the SCR system.
Reactors for generating ammonia gas are most effective when operating within a particular temperature range. Therefore, generation of ammonia gas may be delayed and/or hindered after a cold start of an engine. Accordingly, it may be desirable to provide an aftertreatment system that can effectively provide ammonia gas to an SCR catalyst immediately following a cold start of an engine.